In recent years, water absorbing resins having excellent water absorbing properties have been developed, and have been heavily used in predominantly applications for disposable items as in absorbing articles such as disposable diapers and sanitary napkins, as well as water retention agents for use in agriculture and horticulture water retention agent, industrial water cut-off materials, and the like. In such water absorbing resins, variety of monomers and hydrophilic polymers have been proposed as raw materials. Among them, polyacrylic acid (polyacrylate) based water absorbing resins in which acrylic acid and/or a salt thereof is used as a monomer have been industrially used most often in light of their superior absorption performances.
Conventionally, known water-absorption properties expected for the water absorbing resin as described above involve many characteristics (parameters) such as centrifuge retention capacity, absorbency against pressure, water-absorption speed, liquid permeability without pressure, liquid permeability under pressure, impact resistance, resistance to urine, flowability, gel strength, color, range of particle size distribution and the like. In addition, with regard to the same physical property (e.g., centrifuge retention capacity), many definitions according to various aspects (parameter measurement methods) were proposed.
As water absorbing resins (particulate water absorbing agents) which have been developed while focusing on these many physical properties, those targeted to or specified for these physical properties have been also produced and used. However, even though the aforementioned numerous physical properties (e.g., “centrifuge retention capacity”, “absorbency against pressure” and the like) were controlled, there still exists a problem of hardly achieving sufficient performance in practical applications in absorbent cores such as disposable diapers and the like.
Hereinafter, conventionally targeted physical properties of water absorbing resins and means for achieving them will be explained.
Because water absorbing resins are used generally in applications for disposable items (disposable diapers and the like), they must be necessarily inexpensive. Hence, improvement of productivity has been strongly desired. Also, according to use in absorbing articles, they are expected to be free from problems involving safety and coloring, as a matter of course. More specifically, because unreacted acrylic acid remains in water absorbing resins although in an amount of several hundreds to thousands ppm by weight, lowering of its amount has been desired. Furthermore, since water absorbing resins are combined with white pulp in absorbing articles, they per se are expected to be white colored so as to avoid unusual feeling due to coloring.
Additionally, water absorbing resins have water-swelling property as well as water insolubility. However, as described in Document 1, uncrosslinked water soluble polymer (extractables in water) is included in the water absorbing resin in an amount of several % by weight to several ten % by weight, and these extractables adversely affect water-absorption properties. Therefore, lowering of the amount of the extractables in water has been desired. Moreover, as described in Document 2, physical properties under pressure such as absorbency against pressure and liquid permeable amount under pressure have been desired for water absorbing resins or particulate water absorbing agent (or particulate form water absorbing materials) in absorbing articles.
In attempts to solve the problems as described above, a process in which polymerization is carried out after purification of monomer to give the heavy metal content of not higher than 0.1 ppm (Document 3), a process in which acrylic acid including less acrylic acid dimer oligomer is used (Documents 4, 5), a process in which polymerization is carried out after purification of acrylic acid to give the content of acetic acid and propionic acid of lower than 400 ppm (Document 6), a process in which acrylic acid including less protoanemonen is used (Document 7), a process in which acrylic acid including less furfural is used (Document 8), a process in which acrylic acid including less hydroquinone is used (Document 9) and the like were proposed. Additionally, as the process for lowering impurities in raw materials of a water absorbing resin, a process in which acrylic acid is treated with an aldehyde treating agent (Document 10), a process in which acrylic acid is treated with activated charcoal (Document 11) were proposed.
Although processes for attaining a water absorbing resin having superior physical properties by a method for production in which a raw material, acrylic acid or the like, is highly purified have been proposed as in Documents 3 to 11, there existed problems involving cost and productivity.
Furthermore, for improving physical properties, a process for polymerization of a water absorbing resin in this a certain amount of a minor component is added was proposed. For example, a process in which methoxyphenols included in acrylic acid are adjusted to 10 to 160 ppm (Document 12), a process in which furfural in an amount of 11 to 2000 ppm is allowed to coexist (Document 13), a process in which a metal is used (Documents 14, 15) and the like were proposed. However, according to the process described in Documents 12 and 13, methoxyphenol or furfural included in the monomer which may cause oxidative coupling during the production step of the water absorbing resin could result in a problem of coloring (yellowing) of the water absorbing resin thus obtained.
Moreover, in order to solve the problems described above, a process in which polymerization is carried out with a reversed phase suspension polymerization method using hydroxyperoxide and a reducing agent, followed by a treatment with a silane coupling agent (Document 16), a process in which an organic phosphorus compound is incorporated (Document 17), a process for production in which an inorganic reducing agent such as hypophosphorous acid is incorporated (Document 18), a process in which a treatment with an organic carboxylic acid (salt) is conducted (Document 17), a process in which a treatment with a reductive compound and an inorganic or organic acid is conducted (Document 20), a process in which a sulfinic acid derivative is used as a polymerization initiator (Document 21) were proposed. However, any process involved problems of cost and productivity slowdown. Additionally, when the water absorbing resin was used at a high concentration, there existed problems in absorption properties such as liquid permeability and incorporation of the liquid to be absorbed such as urine
Furthermore, a technique for improving liquid permeability of water absorbing resins in absorbent cores and breathability of the absorbent cores through regulating the average particle diameter to fall within the range of 400 to 850 μm (Document 22) was proposed. However, when the water absorbing resin was used in the absorbent core at a high concentration using this technique, the liquid permeability was elevated, while unsanitary impression may be made to consumers due to coarse particles, and to conspicuous yellowing of the water absorbing resin per se.
Moreover, in addition to the aforementioned procedures, a process for producing a water absorbing resin using an ammonium salt as a counter ion of carboxylic acid in order to improve various physical properties of the water absorbing resin (Document 23 to 25) was proposed. However, there existed problems of coloring (yellowing) of the resulting water absorbing resin, and it was still insufficient in terms of solution of the aforementioned various problems.
Also, heretofore, when the water absorbing resin particles have a small average particle diameter, for example, when the particle diameter is less than 150 μm, in particular, when particles of smaller than 45 μm account for not less than 5% by weight, particularly beyond 10% by weight, such a particulate water absorbing agent inhibits liquid permeability because gel blocking is caused, although a white-colored state resulting from scattering of visible light may be exhibited. Hence, “characteristic of maintaining a white-colored state” and “liquid permeability characteristic” of the particles are contradictory parameters.
(Document 1) U.S. Pat. No. 4,654,039
(Document 2) U.S. Pat. No. 55,662,646
(Document 3) JP-A No. H3-31306
(Document 4) JP-A No. H6-211934
(Document 5) International Publication No. WO04/52949
(Document 6) International Publication No. WO03/95510
(Document 7) European Patent No. 1302485
(Document 8) United States Patent Publication No.
(Document 9) U.S. Pat. No. 6,444,744
(Document 10) International Publication No. WO03/14172
(Document 11) International Publication No. WO04/52819
(Document 12) United States Patent Publication No. 2004/0110914
(Document 13) United States Patent Publication No. 2004/0110897
(Document 14) U.S. Pat. No. 5,439,993
(Document 15) European Patent No. 1457541
(Document 16) JP-A No. H4-331205
(Document 17) JP-A No. H5-86251
(Document 18) U.S. Pat. No. 6,359,049
(Document 19) JP-A No. 2000-327926
(Document 20) JP-A No. 2003-52742
(Document 21) International Publication No.
(Document 22) U.S. Pat. No. 6,617,489
(Document 23) JP-A No. S62-273283
(Document 24) JP-A No. 2004-315816
(Document 25) JP-A No. 2004-323606